In MacOSX, using the standard Mail.app, you can turn on mailbox threaded conversations, where all the mails for a conversation are collapsed into a single row and can be expanded. When using the mutt email client I configured a macro which allows me to tag several mails and then force them collapse into the same thread by setting the Message-ID header into all of them. Does such a plugin exist for MacOSX's mail application? Here's the mutt feature http://firstname.lastname@example.org/msg18310.html by Ulf Erikson.
Exactly what I'm trying to accomplish is select two random messages in my inbox (on IMAP server) and force them into a conversation. Also, select one and break it out of the conversation. I've been using this in mutt for ages in mailing lists, where sometimes people would reply to a thread changing the subject (there I would split the conversation thread). Or when I would receive several automated mails like "You bought this!", "We are billing you", "Here's the bill" collapse them into a conversation even though they are individual and come from different addresses without any filter rule-able pattern.